The invention relates to a method of operating a spark-ignition internal combustion engine, in particular a spark-ignition internal combustion engine with direct fuel injection wherein, in an initial fuel injection event, fuel is injected into the intake air to form in a cylinder a lean charge mixture and, in a main fuel injection event, a stratified rich mixture is established in the vicinity of the spark plug and is ignited by the spark plug.
In gasoline engines with intake pipe injection and in direct-injection gasoline engines operated in a homogeneous operating mode, a reduction in fuel consumption can be obtained by means of internal residual exhaust gas retention or external recirculation of combustion gases. Said reduction is based on so-called dethrottling. This means the reduction of the charge exchange work by means of an increase in the intake pipe pressure. The increase in the intake pipe pressure occurs since the renewed intake of residual exhaust gas increases the overall charge, and therefore a larger gas quantity must pass into the combustion chamber during the intake stroke. The larger the re-circulated residual exhaust gas quantity, the higher is the overall charge in the combustion chamber and also, therefore, the higher the required intake pipe pressure. However, the possible exhaust gas recirculation rate is limited by the combustion stability deteriorating with an increasing exhaust gas content in the combustion gas.
A further advantage of exhaust gas recirculation or retention is the reduction of the combustion temperature as a result of the presence of inert gas during combustion. The inert gas is heated by the hot combustion gas during combustion and correspondingly deprives the combustion gas of energy. The reduced combustion chamber temperature considerably reduces the formation of nitrogen oxides, since the formation of nitrogen oxides is primarily dependent on the temperature. High temperatures and the presence of oxygen promote the formation of nitrogen oxides.
Operating a gasoline engine with a lean charge mixture likewise permits a reduction in fuel consumption. The charge quantity and therefore the intake pipe pressure are also increased as a result of the greater mass of combustion air which is taken in, which, in this case however, comprises only fresh air. The moment generated during combustion is predefined by the injected fuel quantity reduced by the relatively low combustion efficiency of lean combustion.
The formation of nitrogen oxides is promoted by the presence of excess oxygen during combustion. Those nitrogen oxides cannot be reduced to nitrogen and oxygen by a 3-way catalytic converter as a result of the presence of oxygen in the combustion gas, and must be eliminated by means of a NOx storage catalytic converter or similar expensive exhaust gas purification techniques. For this reason, the formation of nitrogen oxides must be minimized during combustion, this being achieved primarily by reducing the combustion chamber temperature. The combustion chamber temperature decreases with an increasing excess of air; a lean operating mode must therefore be implemented with the highest possible excess of air.
It is therefore an object of the present invention to provide a method of operating an internal combustion engine, which method stabilizes the combustion in the combustion chamber and reduces nitrogen oxide emissions even at high residual or re-circulated exhaust gas rates or with a very lean charge mixture.